1. Field of the Invention
The present invention relates to a polyester fiber which is excellent in properties such as dyeability with cationic dyes, easy dyeability under atmospheric pressure at a temperature of about 100.degree. C., resistance to light, resistance to hydrolysis, and a high fiber strength.
2. Description of the Prior Art
Since aromatic polyesters represented by polyethylene terephthalate are excellent in mechanical properties, resistance to light, resistance to heat and resistance to chemicals, they have been widely used as fibers, films, and other shaped articles. However, when the aromatic polyesters are used as fibers, they have to be dyed under high-temperature, high-pressure conditions or have to be subjected to carrier-dyeing. In addition to such economic and operational drawbacks, they have further disadvantages that they are difficult to dye into brilliant colors.
In order to overcome these drawbacks, a polyester which comprises, as its copolymer components, a compound having an acidic group such as 5-sodium sulfoisophtalic acid has been proposed (Japanese Patent Publication No. 34-10497). When dyeing the fiber obtained from this polyester with cationic dyes or disperse dyes, in order that it is sufficiently dyed under atmospheric pressure at a temperature of about 100.degree. C. and without use of carriers, it is necessary that the isophthalic acid component having a metal sulfonate group should be contained in the above polyester in an amount of more than 5 mol% based on the total moles of the dicarboxylic acid components of the copolymer. However, in such a polyester containing a large amount of isophthalic acid component having a metal sulfonate group, the melt viscosity of a polymerized reactant is extremely increased because of the thickening action of the isophthalic acid component during the polymerization reaction, so that the polymerization degree of the polymerized reactant becomes difficult to sufficiently increase. As a result, the mechanical strength of the obtained polyester is decreased, leading to a decrease in the polyester fiber strength and making it difficult to spin. Moreover, the polyester copolymerized with a large amount of the isophthalic acid component having a metal sulfonate group becomes poor in resistance to hydrolysis, so that it is difficult to employ this polyester for practical use.
As a polyester which has dyeability with cationic dyes and is readily dyed under atmospheric pressure at a temperature of about 100.degree. C., a polyester copolymerized with a high molecular weight polyoxyethylene glycol or its homologue having a molecular weight of 200 or more has been proposed (Japanese Laid-Open Patent Publication No. 57-63325). The melting point of this polyester is satisfactory and the fiber obtained from this polyester is excellent in easy dyeability. However, the critical disadvantage of this modified polyester is in that the dyed polyester fiber is poor in resistance to light. As a method to lessen the decrease in resistance to light of the dyed polyester fiber and to improve easy dyeability thereof, a polyester comprising, as dicarboxylic acid components of the copolymer, an aromatic dicarboxylic acid such as adipic acid and isophthalic acid or an alkyl ester of these acids has been proposed (Japanese Laid-Open Patent Publication No. 57-66119). Moreover, a polyester comprising, as diol components, diethylene glycol, 1,4-bis(.beta.-hydroxyethoxy)benzene, bis-ethoxylated 2,2-bis(2,5-dimethyl-4-hydroxyphenyl)propane, neopentyl glycol, or cyclohexanedimethanol, and isophthalic acid component having a metal sulfonate group has been proposed. However, the fiber obtained from this polyester is not readily dyed or even though the fiber is readily dyed, the melting point of the polyester is extremely low. The strength of any one of the above-mentioned polyester fibers is not satisfactory.
On the other hand, as a polyester fiber having a satisfactory fiber strength as well as dye-ability with cationic dyes, a polyester in which the content of the isophthalic acid component having a metal sulfonate group is in the range of 0.8 to 1.8 mol% based on the total moles of the dicarboxylic acid components of the copolymer has been proposed (Japanese Laid-Open Patent Publication Nos. 58-126376, 59-30903, 59-47485, 59-66580, 59-71487, 59-71488, and 59-76987). However, there are the following problems. The dyeability of the fiber obtained from the polyester in which the content of the isophthalic acid components having a metal sulfonate group is 2 mol% or less based on the total moles of the dicarboxylic acid component of the copolymer is decreased, and the fiber cannot be dyed in dark colors. Moreover, a polyester comprising, as its dicarboxylic acid components of the copolymer, an isophthalic acid component having a phosphonium sulfonate group has been proposed (Japanese Patent Publication No. 47-22334 and U.S. Pat. No. 3732183). It is considered that the fiber obtained from this polyester has a high strength and dyeability with cationic dyes. However, this kind of polyester is decomposed by itself under the conditions of high heat during the polymerization step, the molding by fusion step, and the like because of the poor resistance to heat of the phosphonium salt of the isophthalic acid component, so that there arises a critical problem that the quality of the polyester obtained is decreased. Furthermore, dyeability of the above polyester fiber is not sufficient.